Well completion method



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f-LEAD sHleLO u .I f 4g C INVENTOR. l'jv% D JAMES L.R!KE, \7 BY M United States Patent C) M 3,291,207 WELL COMPLETION METHOD James L. Rike, Houston, Tex., assignor, by mesne assignments, to Esso Production Research Company, Houston, Tex., a corporation of Delaware Filed Dec. 19, 1960, Ser. No. 76,715 9 Claims. (Cl. 166-4) The present invention concerns a method of depth control in well operations and particularly in operations in wells containing `a multiplicity of pipe strings. It is especially applicable in multiple tubingless completions. In these type completions a plurality of pipe strings are set in a well bore. The pipe strings and productive formations penetrated by the well bore are perforated, and each formations liuids are produced independently through the individual pipe strings.

In order to obtain `accurate depth control in completions, recompletions, or work-over operations conducted in wells, it is conventional practice to make a radioactivity survey of the well to locate positions of subsurface productive formations, together with a pipe string collar locator log, to indicate positions of the pipe string collars. Correlating positions of the pipe string collars and the positions of the productive formations in this manner permits work tools, eg., perforator guns, to be accurately positioned in the well adjacent any particular `Productive formation by lowering the work tool, together with a pipe string collar locator logging device, the distance determined by the number of pipe string collar indications between the surface ofthe earth and the particular formation as established by the original pipe string collar locator and radioactivity survey correlation log, or the depth is determined through a comparison of the variation in joint lengths indicated by the collar log pips on the correlation log near the zone -of interest and the similar variation obtained on the collar log attached to the work tool or perforating gun. In wells containing two or more spaced-apart, .parallelly extending pipe str-ings, it is necessary to obtain the correlating pipe string collar locator and radioactivity survey log for each pipe string to accurately establish locations of the productive formations relative to locations of the pipe string collars, because the conventional pipe string collar locating device detects only the collars of the pipe string through which it is run. On the other hand, if the collar locator device detected the collars of adjacent pipe strings, there would be no way to distinguish the collars of one pipe string from those of the other pipe strings. Therefore, it is necessary t-o restrict detection of the collars to the pipe string through which the pipe collar locator device 1s run.

In completion operations when more than one pipe string is positioned adjacent a productive formation it is desired to perforate, it is necessary to direct the fire of the gun elements to avoid striking and damaging one or more of the pipe strings other than the pipe string through which the gun is lowered. One method for directing the re of a gun perforator for this purpose utilizes radioactivity detection techniques. In one of these, a radiation detector and -a source of bombarding radiation are positioned in the pipe string through which the uids are to be produced, and the radiation resulting from bombardment by the source radiation is detected, as described and claimed in U.S. patent application Serial No. 780,524, filed Dec. 15, 1958, by Harry S. Arendt, entitled Method and Apparatus for Operating in Wells. In another of these techniques, a radiation detector is arranged in the pipe string to be detected, and a radiation source is positioned in the pipe string through which the fluids are to be produced, and the direct primary radiation from the source is detected, `as described and claimed in U.S. patent 3,291,207 Patented Dec. 13, 1966 ICC application Ser. No. 780,517, filed Dec. 15, 1958, by James T. Brumble, Ir., entitled Method and Apparat-us for Well Operations. Other techniques of this nature are known to those skilled in the art.

When operating in wells containing a multiplicity of pipe strings, it is expensive and time consuming to obtain the required radioactivity survey and pipe string collar locator logs for each pipe string. A procedure which minimizes cost and time consumed in operations of this nature is provided by the present invention. In one technique covered by the invention, at least one marker in the form of a radioactivity shield is used; eg., a lead shield is placed at a desired location or locations along the length of at least one of the pipe strings, preferably the pipe string in which the correlation collar locator and radioactivity log is to be run. When the radioactivity survey is made, the location or locations of the markers are correlated with the locations of the producing formations. To accurately locate the producing formations when operating in the pipe strings other than the pipe string in which the marker has been placed, it is only necessary to use a tool capable of locating the marker and then raise or lower the marker pickup apparatus to the desired formation the distance shown by the radioactivity suivey log. When pipe strings of different lengths are employed, it is preferable to place the markers in the longest pipe string.

This technique is especially useful for completion and recompletion operations in which the perforating gun itself carries a source and detector of radiation. Thus, in this instance, the gun perforator orienting apparatus serves the dual function of orienting the gun perforator and detecting the location of the marker. Once the location of the marker has been established, the gun perforator asasembly is moved upwardly or downwardly the measured distance from the marker as determined from the radioactivity survey log, oriented according to known techniques, and fired.

In another technique covered by the invention, at least one marker is positioned in one or more of the pipe collars of the pipestring or pipe :strings other than the pipe string in which the correlati-on pipe string collar locator and radioactivity survey log is to be made. That is, one of the pipe strings is left unmarked, and the other remaining pipe lstrings are marked with a radioactivity absorbing material. Then, whenthe radioactivity survey of the induced radiation type is made in the unmarked pipe string, the positions of the radioactivity absorbent Imaterials spotted in the collars of the other piper strings are located relative t-o the locations of the producing formations. To accurately position a tool adjacent a particular formation when operating in any one of the marked pipe strings, it is only necessary to lower the tool, along with a pipe string collar locator, t-o adjacent the pipe collar nearest to the depth of the marker as established by the original radioactivity survey and then raise or lower the tool to the particular formation the distance between the marker and the formation as shown Iby the original radioactivity survey log. If desired, all of the pipe strings could be marked in this manner and the same procedure used to position a tool in any of the pipe strings. However, it is easier to position a tool in the pipe string in which the radioactivity survey is taken in accordance with the previously described conventional procedure.

By properly spacing the shielding material in the marked pipe strings, the identity of each can be clearly ascertained. However, to avoid the possibility of confusion as to the identity of a particular pipe string, different lengths of pup joints for each marked pipe string may be strategically located along the lengths of the pipe strings and the shielding material markers placed in the collars or along the lengths of the lpup joints. Thus, in one pipe string, 2-foot pup joints may be arranged. foot pup joints may be used, etc.

Thus, a primary object of this invention is to provide an improved method for more accurately maintaining depth control for well operations conducted in wells containing a plurality of spaced-apart, parallely extending pipe strings.

The above object and other objects of the invention will be apparent from a more detailed description thereof when taken in conjunction with the drawings wherein:

FIG. 1 is a cross-sectional view of the earths subsurface showing a well bore penetrating a plurality of producing formations and containing a plurality of pipe strings, `one of which is provided with radioactivity shielding'markers;

FIG.y 2 is a cross-sectional view of the marked pipe string shown in FIG. 1 showing suspended in it instrumentation for making a radioactivity survey of the well;

FIG. 3 illustrates a representative radioactivity survey log made in the marked pipe string;

FIG. 4 is a cross-sectional view of one of the unmarked pipe strings shown in FIG. 1 showing suspended in it a gun perforator assembly including radioactivity gun orienting elements;

FIG. 5 is a cross-sectional view of the earths subsurface showing a well bore penetrating a plurality of producing for-mations and containing a plurality of pipe strings, two of which are provided with radioactivity shielding markers;

FIG. 6 illustrates a representative pipe collar and radioactivity survey log made in the unmarked pipe string;

FIG. 7 is a cross-sectional view of one of the marked pipe strings shown in FIG. 5 showing suspended in it a .gun perforator assembly including radioactivity `gun orienting elements and also a pipe collar locator device;

FIG. 8 illustrates a pipe string collar log made in one of the marked pipe strings of FIG. 5;

FIG. 9 is a view similar to that shown in FIG. 5 illustrating a modified arrangement of the markers;

FIG. 10 illustrates a representative pipe collar and radioactivity survey log made in the -unmarked pipe string of FIG. 9;

FIG. 11 is a cross-sectional view of one of the pipe strings illustrating one manner of spotting the shield in a pipe string; and

FIG. 12 is a si-milar view illustrating another manner of placing the radioactivity shielding material in a pipe string.

Referring to the drawings in greater detail, in FIG. 1 is shown a borehole 10 penetrating three subsurface productive formations A, B, and C. Three pipe strings 11, 12, and 13 are arranged in borehole 10 and cemented therein. Pipe string 11 is provided with markers 15 spaced along the length of the pipe string. Pipe string 11 extends the length of borehole 10 to adjacent the lowermost formation C. Pipe string 12 extends to adjacent intermediate formation B, and pipe lstring 13 only extends to adjacent the uppermost formation A. Although this particular arrangement has been used to illustrate the invention, if desired, all of the pipe strings may extend the length of the borehole to adjacent formation C. Also, although several markers 15 are shown in pipe string 11, if desired, only one marker may be used. Markers 15, which may be lead-coated joints 15a of the pipe strings as seen in FIG. 12 or leaded sleeves 15b between the joints of the pipe strings as seen in FIG. 13 are preferably spaced apart a few hundred feet and positioned at points or places other than adjacent the productive formations in order to avoid possible confusion during subsequent gun orienting manipulation.

To illustrate the invention, a multiple tubingless completion will be described. The general procedure for practicing the method of Vthe invention in this manner is as follows. Borehole 10 is formed and penetrates'producing formations A, B, and C. Then a conventional electrical resistivity log may be run in the open hole to establish the depth locations of productive formations A, B, and C. Pipe strings 11, 12, and 13 then are run and landed and cemented in borehole 10. Prior to In another pipe string, 4-

running pipe string 11, it is marked with shields 15. Then, a conventional gamma-gamma or gamma-neutron logging tool, such as assembly 16 suspended in pipe string 11 on an electrically conductive cable 17, as seen in FIG. 2, is used to traverse pipe string 11 and log the borehole. The elements of assembly 16 include a radiation detector 18, a shield 19, and a source of radiation 20. Shield 19 prevents the detection of direct radiation emanating from the source 20 by detector 18. In this instance, a lead-coated collar 15 is shown connected in pipe string 11. Cable 17 connects to a recorder 21, which records a radioactivity log 22, as shown in FIG. 3. A collar locator device 23 also is suspended on cable 17. The record of this log is shown at 24 in FIG. 3. Pips 15 on log 22 indicate markers 15 on pipe string 11. The formations A, B, and C are indicated by the change in the log as at A', B', and C. Pipe collars are indicated at 24 on log 24. If it is desired to perforate the lowermost producing formation C, it is only necessary to lower a gun perforator, together with a collar locator device, to the depth of formation C as established by the original correlative radioactivity log 22 and pipe collar locator log 24. That is, the proper depth is reached when the same num'ber of pipe collars between the surface and formation C are counted olf on the pipe collar log run with the gun perforator, or the depth is determined through a comparison of the variation in joint lengths indicated by the collar log pips on the correlation log near the zone of interest and the similar variation obtained on the collar log attached to the work tool or perf-orating gun. Since there are no intervening pipe strings in any direction surrounding pipe string 11 adjacent formation C, it is not necessary to orient the gun perforat-or in any particular direction.

When it is desired `to perforate formations A or B, it is necessary to orient the gun perforator to direct the re thereof .in a direction so as to avoid striking one or more of the adjacent parallel pipe strings. Also, to perforate either of these formations, it is necessary to position the gun perforator at the proper depth. FIG. 4 shows perforator and orienting apparatus which includes a rotatable assembly 25 consisting of gun elements 26, a focused source of radiation 27 (instead of focusing the source of radiation, the detector of radiation could be focused so as to detect radiation in only one direction), a radiation shield 28 designed to prevent the bombarding source of radiation 27 from being detected directly by the detector of radiation 29 positioned above shield 28, and a rotator 30 provided with a plurality of centralizers or restrainers 31 suspended on a wire line 32, which is attached at its upper end to a recorder 33, and fire control mechanism 34. To perforate formation B for example, the assembly 25 is lowered on wire line 32 through pipe string 12 as seen in FIG. 4 to approximately 6600 feet, and then it is raised and lowered while simultaneously rotating the apparatus or assembly until the marker 15 at the position of 6600 feet is located. Once this position has been established, assembly 30 is lowered 40 feet from this location to the 6640-foot mark evidenced on log 22 as the upper level of formation B. Then, gun elements 26 are directed so as to lire in a direction away from pipe string 11 in accordance with known techniques, and the gun is red by the re control mechanism 34 at the surface. Formation A is perforated in a similar manner, except marker 1S at 3900 feet would be used as the reference point, and the gun elements 26 would be directed away from both pipe strings 11 and 12 prior to firing thereof. l

In FIG. 5 three pipe strings 41, 42, and 43 are arranged in Iborehole 10 and cemented therein. Pipe strings 42 and 43 are provided with radioactivity shielding material markers 45 and 46, respectively, strategically positioned near productive formations B and A, respectively. Pipe string 41 extends the length of borehole 10 to adjacent the lowermost formation C; pipe string 42 extends to adjacent intermediate formation B; and pipe string 43 only extends to adjacent the uppermost formation A. As in lthe previously-described embodiment, although this particular arrangement has been used to illustrate the invention, if desired, all of the pipe strings may extend the length of the borehole to adjacent formati-on C. Also, although only one shielding marker 45, 46 is provided in each pipe string 42 and 43, additional markers spaced apart a few hundred feet may be used. It is preferred to place the markers at points other than adjacent the productive formations in order to avoid possible confusion during subsequent gun orienting operations or other operations involving use of radioactive materials. The radioactivity shielding markers are placed or spotted in or on the collars of the pipe strings, as seen for example in FIG. ll wherein the lead-coated collar 15a is shown.

To illustrate this embodiment of the invention, a multiple tubingless completion operation again will be described. The general procedure for practicing the method of this embodiment of the invention is as follows. Borehole lt) is formed and penetrates producing formations A, B, and C. Then, a conventional electrical resistivity log may be run in the open hole to establish the depth locations of productive formations A, B, and C. Pipe strings 41, 42, and 43 then are run and landed and cemented in borehole l0. Pipe strings 42 and 43 may be equipped with the radio-activity shielding markers 45 and 46, respectively, prior to running the pipe strings into borehole 1t). However, if the radioactivity shielding material is not placed in the pipe collars prior to running of the pipe strings, then temporary radioactivity shielding markers may be positioned in the particular pipe collars. Those selected are preferably adjacent the formation the uids of which are to be produced through that pipe string. As an example of a temporarily positioned marker, a collar stop device with a large compact mass of lead attached may be positioned in and retrieved from a pipe collar by means of Wire line tools. Once the marker has been placed in the particular pipe collar of interest, whether temporarily or permanently, a pipe collar locator logging tool and a conventional gamma-gamma or gamma-neutron logging tool, such as assembly 25a modified by inclusion of a collar locator device 47 as seen in FIG. 7, is used to traverse pipe string 41 and log the borehole. In this instance, recorder 33 records a pipe collar log 48 and a radioa-ctivity log 49, as shown in FIG. 6. Pips 48 on pipe collar log 48 indicate the joints or collars of pipe string 41. On the radioactivity survey log 49 marker 46 is indicated by pip 46', and the marker 45 in pipe string 42 is indicated by the pip 45. Again the formations A, B, and C are indicated by the change in the log as at A', B', and C'.

The logs of FIG. 6 are made by the instrumentation shown in FIG. 7, and once these logs have been recorded, the shielding materials 45 and 46 are no longer needed. Thus, they may be but temporarily placed in the pipe collars.

The same method may be used to perforate the lowermost producing formation C as was described with regard to the embodiment of FIGS. 1-4. However, when it is desired to perforate formations A or B, it is necessary to orient the gun perforator to direct the fire thereof in a direction so as to avoid striking one or more of the adjacent parallel pipe strings, and it is necessary to position the gun perforator at the proper depth to perforate either .of these formations. Thus, to perforate formation B, the assembly 25 and collar locator 47 of FIG. 7 are lowered on Wire line 32 in pipe 42 to a depth of approximately 6600 feet, which is the depth of the marker 45 as shown at 45 on the radioactivity survey log 49.

Also, collar detector 47 records on recorder 33 the pipe collar log 50 shown in FIG. 8. Pips 50 indicate the collars of pipestring 42. It is seen from this log that there is a collar located at 6592 feet and another collar at 6628 feet. Since the collar at 6592 feet is closest to the depth of 660() feet established as the marker 45 on the radioactivity survey log 49, it must be the collar in which the radioactive shielding marker was placed. When this has been established, assembly 30 is lowered 90 feet (6690 minus 6600 feet) as determined from the radioactivity log 49 to formation B. Then the gun elements are di-rected so as to tire in a direction away from pipe string 41 according to known techniques, and the gun is red by the fire control 34 at the surface. Formation A is perforated in a similar manner, except an upper pipe collar would be used as a reference, and the gun elements 26 would be directed away from both pipe strings 41 and 42 prior to firing thereof.

FIGS. 9 and l0 illustrate a similar operation as that described for FIGS. 5-8 inclusive. However, the pipe strings are more positively identified by employing pup joints of different lengths for different pipe strings. In FIG. 9 borehole liti has arranged in it pipe strings 41, 42a, and 43a. As seen more clearly in FIG. 12, the shielding material 15b is a sleeve covering the length of the pup joint. In pipe string 42a the shield 45a may be 4 feet in length, and in pipe string 43a the shielding material 46a may be 8 feet in length. The difference in the lengths of the joints permits ready distinction among the pipe strings, The correlative pipe collar log and radioactivity survey log taken by the instrumentation shown in FIG. 7 are illustrated in FIG. l0. As before, logs 4S and pips 48 indicate the positions of the collars of pipe string 41. The radio-activity survey log 62 is different from the radio-activity survey log 49 of FIG. 6, and in the former is shown pip 46a', which evidences the S-foot joint 46a of pipe string 43a, and pip 45a', which evidences the 4-foot pipe joint 45a of pipe string 42a. The operation is the same with regard to this embodiment; however, by using pup joints of different lengths, the identity of the pipe string is more positively established, and there should be no question about which pipe string the operations are being conducted in.

The sources of radiation for elements 20 and 27 may be fast neutrons or gamma rays. The neutron source may be the result of an alpha neutron, deuteron neutron, or proton neutron reaction in which the alpha particle, deuteron, or proton is accelerated by an electric field and caused to interact with selected target materials in order to produce neutrons of various energies. Or the source may be neutrons originating from radium beryllium or polonium beryllium. Also, high energy gamma radiation sources which may be employed are radioactive Na 24, La 140, Sb 124, Co 60, or high-energy gamma rays produced by various reactions in high-energy particle machines in manners well known to the art of nuclear physics. Detectors 18 and 29 can be used to detect slow neutrons or gamma rays or fast neutrons. For the detection of these types of radiation, ionizing chambers, Geiger-Muller counter tubes, and scintillation counters may be used. The moderating shields 19 and 28 are formed of radiation absorbing or moderating material, such as lead, tungsten, paraffin, boron, cadmium, etc., which materials are capable of absorbing the radiation impinging on it. Similarly, markers 15, 45 and 46, 45a and 46a may be formed of these materials.

Having fully described the method, apparatus, and objects of my invention, I claim:

1. A method for perforating in a Well penetrating at least two vertically spaced-apart, upper and lower productive formations and containing at least two juxtaposed pipe strings, one of which contains at least one detectable marker consisting of a radioactivity absorbing substance positioned 'along the length thereof comprising: running in said one pipe string a radioactivity survey tool .together with a collar locator tool and preparing a correlation log showing the locations of said productive formations, the locations of the pipe collars of said one pipe string, and the position of said marker relative to each other; lowering in said other pipe string a device provided with a perforator having a selected direction of perforation, and with radiation means having a selected direction of pipe detection including a source of radiation and a detector of radiation resulting from interactions of said source radiation, and with means for causing rotation of said device, to the depth of said marker by detection of said marker by said device; moving said device the distance between said marker and said upper formation as shown on said correlation log to position said device adjacent said upper formation; rotating said device, to locate the position of said one pipe string relative to a reference position of said device, and accordingly, to said direction of pipe detection and said direction of perforation with said radiation means; further rotating said device to direct the direction of perforation away from said one pipe string; and -then actuating said perforator to perforate said upper formation.

2. A method as recited in claim 1 including following making of said correlation log, lowering a perforator in said one pipe string to the depth of said lower formation as determined by said correlation log; and actuating said perforator to perforate said lower formation.

3. A method of depth control for a well operation in a well penetrating at least two, productive formations and containing at least two juxtaposed pipe strings cornprising: temporarily positioning in at least one pipe collar of one pipe string a detectable marker consisting of a radioactivity absorbing substance; running in said other pipe string a radioactivity survey tool together with a collar locator tool and preparing therefrom a correlation log showing the locations of said formations, the locations of the pipe collars of said other pipe string, and the position of said marker relative to each other; removing said marker from said one pipe string; running in said one pipe string a well tool together with a collar locator tool and preparing a log showing the pipe collars of said one pipe string; moving said well tool and said collar locator tool vertically to the pipe collar in said one pipe string nearest in depth to the depth of said marker as shown on said correlation log; and then moving said well tool and said collar locator tool the distance between one of said formations and said marker as shown on said correlation log to position said well tool in said one pipe string adjacent said one formation.

4. A method as recited in claim 3 in which said well operation is perforating and said well tool is a perforator and including actuating said perforator to perforate said one formation after positioning said well tool adjacent said one formation.

5. A method as recited in claim 3 including employing as said marker a collar stop device having a mass of lead attached thereto.

6. A method of depth control for a well operation in a well penetrating at least two productive formations and containing at least two juxtaposed pipe strings, one of which contains in at least one pipe collar thereof a detectable marker consisting of a radioactivity absorbing substance comprising: running in said other pipe string a radioactivity survey Itool together with a collar locator tool and preparing therefrom a correlation log showing the locations of said formations, the locations of the pipe collars of said other pipe string, and the position of said marker relative to each other; running in said one pipe string a well tool together with a collar locator tool and preparing a log showing the pipe collars of said one pipe string; moving said well tool and said collar locator tool vertically to the pipe collar in said one pipe string nearest in depth to the depth of said marker as shown on said correlation log; and then moving said well tool and said collar locator tool the distance between one of said formations and said marker as shown on said correlation log to position said well tool in said one pipe string adjacent said one formation.

7. A method as recited in claim 6 in which said well operation is perforating and said well tool is a perforator and including actuating said perforator to perforate said one formation 4after positioning said well tool perforator adjacent said one formation.

8. A method as recited in claim 6 including employing as said marker a lead coated pipe collar.

9. A method of depth control for perforating in a well penetrating at least three spaced-apart, upper intermediate and lower productive formations and containing at least three, first, second and third juxtaposed pipe strings, at least two of which, viz., said second and third pipe strings contain in at least one pipe collar an identitiable detectable marker consisting of a radioactivity absorbing substance comprising: running in said first pipe string a radioactivity survey tool together with a collar locator tool and preparing therefrom a correlation log showing the locations of said formations, the locations of the pipe collars of said tirst pipe string, and the positions of said markers of said second and third pipe strings, relative to each other; lowering a perforator in said first pipe string to the depth of said lower formation as determined from said correlation log; actuating said perforator to perforate said lower formation in order to produce said lower formation independently through said first pipe string; running a well tool in said second pipe string together with a collar locator tool, said well tool including a perforator having a selected direction of perforation and radiation means having a selected direction of pipe detection including a source of radiation and a detector of radiation resulting from interactions of said source radiation, and means for causing rotation of said tool, and preparing a log showing the pipe collars of said second pipe string; moving said well tool and said collar locator ltool vertically to the pipe collar in said second pipe string nearest in depth to the depth of said marker of said second pipe string as shown on said correlation log; further moving said well tool and said collar locator tool the distance between said intermediate formation and said marker of said second pipe string as shown on said correlation log to position said well tool adjacent said intermediate formation; rotating said well tool to locate with said radiation means the radial positions of said first and third pipe strings relative to the direction of perforation; further rotating said well tool to direct the direction of perforation of said perforator away from said first and third pipe strings; actuating said perforator to perforate said intermediate formation in order to produce said intermediate formation independently through said second pipe string; running in said third pipe string said well tool and said collar locator tool and preparing a log showing the pipe collars of said third pipe string; moving said well tool and said collar locator tool vertically to the pipe collar in said third pipe string nearest in depth to the depth of said marker of said third pipe string as shown on said correlation log; further moving said well tool and said collar locator tool the distance between said upper formation and said marker of said third pipe string as shown on said correlation log to position said well tool adjacent said upper formation; rotating said well tool to locate with said radiation means the radial positions of said first and second pipe strings relative to the direction of perforation; further rotating said well tool to direct the direction of perforation away from said rst and second pipe strings; and then actuating said perforator to perforate said upper formation in order to produce said upper formation independently through said third pipe string.

References Cited by the Examiner UNITED STATES PATENTS (Other references on following page) 9 UNITED STATES PATENTS Piety Z50-83.6 Russell 166-66 X Segesman 166-4 Doll 166-4 D011 166-4 McKay 175--41 X 10 2,842,852 7/1958 Tanguy 166-4 X 2,871,946 2/1959 Bigelow 166-64 2,998,068 8/1961 True 166-55 5 CHARLES E. OCONNELL, Primary Examiner.

BENJAMIN BENDE'IT, Examiner.

C. D. JOHNSON, D. H. BROWN, Assistant Examiners. 

6. A METHOD OF DEPTH CONTROL FOR A WELL OPERATION IN A WELL PENETRATING AT LEAST TWO PRODUCTIVE FORMATIONS AND CONTAINING AT LEAST TWO JUXAPOSED PIPE STRINGS, ONE OF WHICH CONTAINS IN AT LEAST ONE PIPE COLLAR THEREOF A DETECTABLE MARKER CONSISTING OF A RADIOACTIVITY ABSORBING SUBSTANCE COMPRISING: RUNNING IN SAID OTHER PIPE STRING A RADIOACTIVITY SURVEY TOOL TOGETHER WITH A COLLAR LOCATOR TOOL AND PREPARING THEREFROM A CORRELATION LOG SHOWING THE LOCATIONS OF SAID FORMATIONS, THE LOCATIONS OF THE PIPE COLLARS OF SAID OTHER PIPE STRING, AND THE PORTION OF SAID MARKER RELATIVE TO EACH OTHER; RUNNING IN SAID ONE PIPE STRING A WELL TOOL TOGETHER WITH A COLLAR LOCATOR TOOL AND PREPARING A LOG SHOWING THE PIPE COLLARS OF SAID ONE PIPE STRING; MOVING SAID WELL TOOL AND SAID COLLAR LOCATOR TOOL VERTICALLY TO THE PIPE COLLAR IN SAID ONE PIPE STRING NEAREST IN DEPTH TO THE DEPTH OF SAID MARKER AS SHOWN ON SAID CORRELATION LOG; AND THEN MOVING SAID WELL AND SAID COLLAR LOCATOR TOOL THE DISTANCE BETWEEN ONE SAID FORMATIONS AND 